Inverter circuit for lighting discharge lamps with reduced power consumption

ABSTRACT

An inverter circuit for lighting discharge lamps with reduced power consumption is disclosed. The inverter circuit comprises: a transformer having a resonant circuit formed by a parasitic capacitance of a discharge lamp; an H-bridge circuit to drive a primary side of the transformer at a frequency which is less than a series resonant frequency of the resonant circuit, and at which phase difference in voltage and current at the primary side of the transformer falls within a predetermined range from its minimum; a logic circuit to produce, based on an output signal of an oscillating circuit, gate signals for driving the H-bridge circuit; and a step-up circuit to step up a DC supply voltage (Vcc) based on another output signal of the oscillating circuit, and to supply the logic circuit with the stepped up DC voltage as a supply voltage for producing the gate signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inverter circuit for lightingdischarge lamps for use in a liquid crystal display unit, and the like,and particularly to an inverter circuit with a high power efficiency.

2. Description of the Related Art

In some conventional inverter circuits for lighting discharge lamps, aresonant circuit may be formed by leakage inductance at the secondaryside of a transformer and by parasitic capacitance in a discharge lampconnected as load, and the primary side of the transformer may be drivenby a resonant frequency of the resonant circuit thus formed. An exampleof such inverter circuits is disclosed in U.S. Pat. No. 6,114,814. Sucha conventional inverter circuit to drive the primary side by theresonant frequency involves phase difference in voltage and current atthe primary side of the transformer consequently failing to achieve afavorable power efficiency.

In order to cope with the problem described above, Japanese PatentApplication Laid-Open No. 2003-168585 discloses an inverter circuit fordischarge lamps, in which a transformer is driven in a frequency rangewhere phase difference in voltage and current at the primary side of thetransformer is small thereby providing a high power efficiency, wherebythe power efficiency of the transformer is improved. The invertercircuit for discharge lamps disclosed in the aforementioned JapanesePatent Application Laid-Open No. 2003-168585 comprises: a transformerwhere a resonant circuit is formed by parasitic capacitance in adischarge lamp and auxiliary capacitance; and an H-bridge circuit wherethe primary side of the transformer is driven at a frequency which isless than a series resonant frequency of the resonant circuit, and atwhich phase difference in voltage and current at the primary side of thetransformer falls within a predetermined range from its minimum, thusthe power efficiency is improved.

In an inverter circuit for discharge lamps used in a liquid crystaltelevision (TV) which is one example of liquid crystal display (LCD)units, a supply voltage ranges from 12 to 24V. For example, in aseparate driving inverter which is described in connection with theaforementioned inverter circuit disclosed in Japanese Patent ApplicationLaid-Open No. 2003-168585, and which uses a leakage magnetic flux typetransformer, an inverter control IC to constitute a control section ofthe inverter circuit is operated by a supply voltage of 5.0V, and anH-bridge circuit with an FET to drive a transformer thereby lightingdischarge lamps is operated by a voltage of 12 to 24V.

Recently, a liquid crystal TV is increasing in its screen size, and asmany as 8 to 24 discharge lamps are employed in one liquid crystal TV,and also the length of discharge lamps is increased to, for example,1300 mm. This results in increasing the power consumption up to 180 W.Accordingly, in case of a large-sized liquid crystal TV, its invertercircuit and its discharge lamps are responsible for most of the powerconsumption, and therefore the inverter circuit is required to befurther improved in efficiency for reducing its power consumption.

In order to answer the above-described requirement for improvedefficiency of an inverter circuit for discharge lamps, there is providedan inverter circuit in which a voltage supplied to the H-bridge circuitfor lighting discharge lamps is increased from conventional 12 to 24V upto, for example, 120V. Since, current flowing in the FET can be reduceddue to the increased supply voltage in the inverter circuit, loss due toon-resistance of the FET can be reduced, and also since current flowingin a primary winding of a transformer can be reduced, copper loss can bereduced. Thus, its efficiency is improved. Here, two supply voltages areinvolved: one is 120V supplied to the H-bridge for lighting thedischarge lamps, and the other is 5V supplied to the inverter controlIC. The withstand voltage of the FET of the H-bridge must be increased,and a high gate source voltage is required to drive the FET with a highwithstand voltage. For example, if the withstand voltage of the FET ofthe H-bridge is set at 200V, the gate source voltage of the FET of theH-bridge must be 10V or higher. Consequently, the FET cannot be drivenby a voltage of 5V supplied to the inverter control IC if used as it issupplied, and the voltage supplied must be stepped up by a dischargepump, a bootstrap, or a step-up DC-to-DC converter in order to dulydrive the FET.

However, employing a step-up circuit like the aforementioned dischargepump, bootstrap, or step-up DC-to-DC converter complicates the circuitstructure and increases the number of components. Also, another problemis that there is difference between frequency of an oscillating circuitto operate the H-bridge circuit and frequency of another oscillatingcircuit to operate the step-up circuit, which produces interference at areference voltage of the inverter control IC thus interrupting a stableoperation of the circuit.

SUMMARY OF THE INVENTION

The present invention has been made in light of the above problems, andit is an object of the present invention to provide an inverter circuitfor lighting discharge lamps, which is simply structured, enables stableoperation of a circuit, and which has a further enhanced efficiencythereby reducing power consumption.

In order to achieve the object, according to one aspect of the presentinvention, an inverter circuit for lighting discharge lamps comprises: atransformer having a resonant circuit formed by a parasitic capacitanceof a discharge lamp; an H-bridge circuit to drive a primary side of thetransformer at a frequency which is less than a series resonantfrequency of the resonant circuit, and at which phase difference involtage and current at the primary side of the transformer falls withina predetermined range from its minimum; a logic circuit to generate,based on an output signal of an oscillating circuit, gate signals fordriving the H-bridge circuit; and a step-up circuit to step up a DCsupply voltage based on another output signal of the oscillatingcircuit, and to supply the logic circuit with the stepped up DC supplyvoltage as a supply voltage for generating the gate signals. Thus, thestep-up circuit does not require an oscillating circuit dedicatedthereto reducing the number of components, whereby the high withstandvoltage FET of the H-bridge circuit can be controlled by a simplifiedcircuitry with reduced cost. Consequently, the supply voltage to theH-bridge circuit can be increased therefore enabling current flowing inthe FET to be decreased thus reducing loss due to on-resistance of theFET with the simplified circuitry. Also, since the step-up ratio of thetransformer can be decreased, current at the primary side of thetransformer can be decreased and therefore copper loss can be reduced,whereby efficiency can be improved for reduction in power consumption.Further, since one oscillating circuit is provided for common use,interference generated at reference voltage is prevented thus achievinga stable circuit operation.

In the aspect of the present invention, the oscillating circuit may beformed by the parasitic capacitance of the discharge lamp and anauxiliary capacitance parallel-connected to the discharge lamp.Consequently, a desired oscillating frequency can be achieved easilyaccording to the auxiliary capacitance.

In the aspect of the present invention, the step-up circuit maycomprise: an error amplifier to output a voltage in accordance with anoutput voltage of the step-up circuit; and a PWM circuit to output apulse voltage having a pulse width according to the voltage outputtedfrom the error amplifier based on the output signal from the oscillatingcircuit. Consequently, a constant and stable voltage can be easilyoutputted.

In the aspect of the present invention, the step-up circuit may furthercomprise a slow-start circuit connected to the PWM circuit.Consequently, a transitional excess voltage is prevented from beinggenerated at an output of the step-up circuit.

In the aspect of the present invention, the slow-start circuit providedin the step-up circuit may have a shorter rise time than a slow-startcircuit to start the H-bridge circuit. Consequently, the logic circuitcan rise up stably, and therefore the H-bridge connected to the logiccircuit can also rise up stably.

And, in the aspect of the present invention, the inverter circuit mayfurther comprise a protection circuit to stop an operation of thestep-up circuit when detecting an abnormal circumstance at a side of thetransformer provided with the discharge lamp.

Further, since a reference voltage circuit to supply circuits withreference voltages required by the circuits, a stable inverter circuitfree from malfunction can be provided. And, in the inverter circuitaccording to the present invention, the transformer is driven at afrequency lower than a resonant frequency therefore avoiding influenceof a high order frequency, which makes it to easier to design atransformer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an inverter circuit for lighting dischargelamps, according to an embodiment of the present invention;

FIG. 2 is a graph showing a frequency characteristic of admittance /Y/of a primary side of a transformer when a resonant circuit is formed ata secondary side thereof in the inverter circuit of FIG. 1, and showinganother frequency characteristic of phase difference 0 in voltage andcurrent in the inverter circuit of FIG. 1;

FIG. 3 is a block diagram of a step-up circuit in the inverter circuitof FIG. 1;

FIG. 4 is a waveform chart of output signals of respective slow-startcircuits used in the step-up circuit and a PWM circuit in the invertercircuit of FIG. 1;

FIGS. 5A to 5E are operation timing charts on the inverter circuit ofFIG. 1;

FIGS. 6A to 6F are timing charts of gate signals in the inverter circuitof FIG. 1; and

FIG. 7 is an explanatory chart of an operation of a protection circuitin the inverter circuit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will hereinafter bedescribed with reference to the accompanying drawings.

A block diagram of an inverter circuit for discharge lamps according toan embodiment of the present invention is shown in FIG. 1. For easierunderstanding, an explanation will be first made on a case where apredetermined voltage Va of a terminal 28 a is not applied to aninverting input terminal 11 a of an error amplifier 11 thus lightmodulation does not occur.

An output of a chopping wave 7 of an oscillating circuit 4 is inputtedto a PWM circuit 8. A discharge lamp 9 for backlighting a liquid crystaldisplay (LCD) is disposed in an LCD unit 2 provided at a secondary sideof a transformer 1 (in practice, a plurality of discharge lamps andtransformers are used, but only one each thereof is illustrated for thepurpose of explanation), and its voltage 9 a is inputted to theaforementioned inverting input terminal 11 a of the error amplifier 11by means of a current-to-voltage converter circuit 10 which convertscurrent flowing in the discharge lamp 9 into voltage. A seriesoscillating circuit is formed by parasitic capacitance 3 at thedischarge lamp 9, capacitors 31 and 32 connected to the discharge lamp 9in parallel, and leakage inductance of the transformer 1. The capacitors31 and 32 function as auxiliary capacitance for the parasiticcapacitance 3.

The error amplifier 11 outputs to the PWM circuit 8 an output voltage 12according to the current of the discharge lamp 9, and the PWM circuit 8compares the chopping wave 7 and the output voltage 12 of the erroramplifier 11 and inputs a pulse signal 13 to a counter circuit 14.

A slow-start circuit 34 outputs to the PWM circuit 8 an output signal ofa start driving signal 56 for comparatively gentle rise-up, therebypreventing generation of an instantaneous overvoltage at the time ofstart.

The chopping wave 7 which is an output signal of the oscillating circuit4 is determined by values of a resistor 5 and a capacitor 6, and anoutput pulse signal 16 of the oscillating circuit 4, which issynchronized with the chopping wave 7, is inputted to counter circuits14 and 15, and a logic circuit 29. According to the output pulse signal16 of the oscillating circuit 4 and output pulse signals of the countercircuits 14 and 15, the logic circuit 29 powered by a supply voltage 76of 10V supplied from a step-up circuit 100 generates gate signals 18,19, 20 and 21 with a pulse amplitude of 10V, which are to be inputted toan H-bridge circuit 17.

The H-bridge circuit 17 is structured such that a series circuitconsisting of PMOS (A1) and NMOS (B2) and a series circuit consisting ofPMOS (A2) and NMOS (B1) are connected to each other in parallel, andoperates according to the gate signals 18, 19, 20 and 21. A DC supplyvoltage Vb of 120V for lighting the discharge lamp 9 is converted by thegate signals 18, 19, 20 and 21 with a pulse amplitude of 10V in theH-bridge circuit 17, and lights the discharge lamp 9 through thetransformer 1.

Accordingly, when a burst circuit 22 does not operate thereby notallowing the predetermined voltage Va from the terminal 28 a be appliedto the inverting input terminal 11 a of the error amplifier 11, light isnot modulated, and the current of the discharge lamp 9 is inputted tothe inverting input terminal 11 a, thus the discharge lamp 9 isfeedback-controlled and lighted.

Referring to FIG. 2, an AC current within a frequency indicated by Aflows at the primary side of the transformer 1, and a constant currentcontrol is accomplished within a high power efficiency range to lightthe discharge lamp 9 shown in FIG. 1.

A discussion will now be made on an operation of the step-up circuit100.

The step-up circuit 100 steps up a DC supply voltage Vcc of 5V, andsupplies the stepped up DC voltage to the logic circuit 29 as theaforementioned supply voltage 76. The chopping wave 7, which is theoutput signal from the oscillating circuit 4, and which is used forcontrolling the H-bridge circuit 17, is inputted also to the step-upcircuit 100.

Referring to FIG. 3, the aforementioned DC supply voltage Vcc of 5V isapplied to the step-up circuit 100, is stepped up by a step-up typechopper circuit formed by a transistor 73 to operate on the choppingwave 7, an inductor 74, and a diode 77, then is smoothed by a capacitor78 into a DC voltage of 10V, and is outputted from the step-up circuit100 as the DC supply voltage 76 for the logic circuit 29.

In the step-up circuit 100, PWM control is performed by using an erroramplifier 71 and a PWM circuit 72, and a constant voltage output isachieved. An output voltage of the step-up circuit 100 is detected byresistors 81 and 82, and is compared with a reference voltage Ve by theerror amplifier 71 which then outputs a voltage according to the outputvoltage of the step-up circuit 100. In the PWM circuit 72, the output ofthe error amplifier 71 is compared with the chopping wave 7 outputtedfrom the oscillating circuit 4, and a pulse signal whose pulse width isfeedback-controlled is outputted from the PWM circuit 72. This pulsesignal makes the transistor 73 undergo switching, thereby outputting theDC supply voltage 76 of a constant voltage. Thus, the logic circuit 29is provided with the supply voltage 76 and thereby enabled to output thegate signals 18, 19, 20 and 21 of a high voltage capable of driving anFET of high withstand voltage used in the H-bridge circuit 17.

Since the chopping wave 7 outputted from the oscillating circuit 4 isused in common for controlling the H-bridges circuit 17 and the step-upcircuit 100, and shared by the both circuits, the step-up circuit 100does not need to have an independent oscillating circuit dedicatedthereto thus simplifying the circuitry of the step-up circuit 100. Also,since the H-bridge circuit 17 and the step-up circuit 100 share the useof the chopping wave 7 outputted from the oscillating circuit 4, theoperating frequencies of the both circuits coincide with each other,whereby interference which occurs at a reference voltage when operatingfrequencies differ from each other can be avoided thus eliminating aninstable circuit operation and ensuring a stable circuit operation.

A slow-start circuit 75 outputs to the PWM circuit 72 a signal tocommand comparatively gentle rise-up at the start of operation of thestep-up circuit 100 so that the pulse signal outputted from the PWMcircuit 72 is kept from having a too large width to thereby preventgeneration of transitional excess voltage at the output of the step-upcircuit 100.

Referring to FIG. 4 showing a waveform chart of output signals ofslow-start circuits 75 and 34 used in the step-up circuit 100 and thePWM circuit 8, respectively, in the inverter circuit of FIG. 1, a risetime T1 of the slow-start circuit 75 used in the step-up circuit 100 isset to be shorter than a rise time T2 of the slow-start circuit 34 usedin the PWM circuit 8 so that the logic circuit 29 is allowed to rise upby the slow-start circuit 34 only after the supply voltage 76 isstabilized, whereby the logic circuit 29 can rise up stably, andtherefore the H-bridge circuit 17 connected to the logic circuit 29 canalso rise up stably.

An operation of the burst circuit 22 performing light control of thedischarge lamp 9 will be described with reference to FIGS. 1 and 5A to5E. Referring to FIG. 1, the burst circuit 22 can be set up in either oftwo modes: one mode is such that a resistor 23 has its resistance set ata predetermined value or more whereby a predetermined pulse signal 24inputted to a DUTY terminal 24 a is outputted from the burst circuit 22as a first burst signal 25 b (refer to FIG. 5D); and the other mode issuch that the resistor 23 has its resistance set at less than apredetermined value whereby a chopping wave voltage 27 (refer to FIG.5B) determined by the resistor 23 and a capacitor 26 is compared with aDC voltage 36 (refer to 5B) inputted to the DUTY terminal 24 a therebyoutputting a second burst signal (pulse wave) 25 a (refer to FIG. 5C).

When the first burst signal 25 b from the burst circuit 22 is “H”, atransistor 28 is turned on causing the error amplifier 11 to output tothe PWM circuit 8 an output voltage 12 in accordance with current in thedischarge lamp 9, whereby an output (refer to 5E) of the H-bridgecircuit 17 is formed based on the chopping wave 7 shown in FIG. 5A,which puts the discharge lamp 9 into operation. When the first burstsignal 25 b from the burst circuit 22 is “L”, the transistor 28 isturned off causing the inverting input terminal 11 a of the erroramplifier 11 to be pulled up to the predetermined voltage Va supplied tothe terminal 28 a, whereby the error amplifier 11 is put innon-operation causing the H-bridge circuit 17 to stop its operation,which puts the discharge lamp 9 in non-operation. Thus, the dischargelamp 9 is caused to operate intermittently by the first burst signal 25b, and light control is performed. In this connection, when the secondburst signal 25 a is used, the discharge lamp 9 has it light controlledin the same manner, which allows selective usage of the first and secondburst signals 25 b and 25 a.

The gate signals 18 (refer to FIG. 6B) and 19 (refer to FIG. 6C), whichare both formed at the logic circuit 29 by the supply voltage 76 fromthe step-up circuit 100, and which have a pulse amplitude of 10V,alternately rise up respectively at each upper peak 18 u and 19 u (referto FIG. 6A) of the chopping wave 7 by means of counter circuits 14 and15, and the logic circuit 29, and alternately fall down respectively ateach cross point 18 d and 19 d (refer to FIG. 6A) of the chopping wave 7and the output signal 12 of the error amplifier 11. Gates of the PMOS(A1) and the PMOS (A2) rise up and fall down respectively by the gatesignals 18 and 19 having a pulse amplitude of 10V.

Also, the gate signals 20 (refer to FIG. 6D) and 21 (refer to FIG. 6E),which are both formed at the logic circuit 29 by the supply voltage 76from the step-up circuit 100, and have a pulse amplitude of 10V,alternately rise up respectively at each lower peak 20 u and 21 u (referto FIG. 6A) of the chopping wave 7 by means of the counter circuits 14and 15, and the logic circuit 29, and alternately fall down respectivelyat cross each point 20 d and 21 d (refer to FIG. 6A) of the choppingwave 7 and the output signals 12 of the error amplifier 11. Gates of theNMOS (B1) and the NMOS (B2) rise up and fall down respectively by thegate signals 20 and 21 having a pulse amplitude of 10V.

Referring to FIGS. 6B to 6E, the gate signals 21 and 20 rise up behindthe gate signals 18 and 19, respectively, and referring to FIG. 6F, thegate signals 18 and 19 fall down behind the gate signals 21 and 20,respectively, by a time t1 predetermined by a delaying circuit 35.Consequently, the PMOS (A1) PMOS (A2) and the NMOS (B1)/NMOS (B2) do notturn on concurrently. Thus, the gate signals 18, 19, 20 and 21 which donot allow the PMOS (A1)/PMOS (A2) and the NMOS (B1) NMOS (B2) to turn onconcurrently can be easily formed by the chopping wave 7 and the outputvoltage 12.

An error amplifier 51 for voltage feedback compares an applied voltagesignal 55 of the discharge lamp 9 inputted to an inverting inputterminal 51 a with a preset value Vc, and outputs to a protectioncircuit 50 and the PWM circuit 8 an output voltage 52 in accordance withthe voltage applied to the discharge lamp 9. The protection circuit 50incorporates a comparator circuit (not shown), to which the outputvoltage 52 from the error amplifier 51 for voltage feedback, and atransformer output current signal 53 from a resistor 57 provided inseries with the secondary side of the transformer 1 are inputted. Theapplied voltage signal 55 is formed such that a voltage at a connectionof the capacitors 31 and 32 disposed at the output side of thetransformer 1 is divided by resistors 58 and 59.

The error amplifier 51 for voltage feedback, when the applied voltagesignal 55 is inputted to its inverting input terminal 51 a, compares theapplied voltage signal 55 with the preset value Vc, and outputs theoutput voltage 52 to the PWM circuit 8, and the voltage applied to thedischarge lamp 9 is feedback-controlled. Accordingly, for example, whenthe discharge lamp 9 is not connected or poorly connected, an openvoltage can be defined as a preset value. Also, when the discharge lamp9 is not connected or poorly connected, it can happen that the outputvoltage at the secondary side of the transformer 1 shows an abnormalvalue. In such a case, the output voltage 52 of the error amplifier 51for voltage feedback inputted to the protection circuit 50, and thetransformer output current signal 53 are compared with the referencevoltage of the comparator circuit (not shown) of the protection circuit50, and if the output voltage 52 of the error amplifier 51 or thetransformer output current signal 53 exceeds the reference voltage, thenthe logic circuit 29 is caused to stop its operation, whereby an excesscurrent to the discharge lamp 9, and an excess voltage to thetransformer 1 can be prevented. Further, the protection circuit 50, whenthe output voltage 12 of the error amplifier 11 is inputted, functionsto prevent an excess current to the discharge lamp 9 and an excessvoltage to the transformer 1. Thus, when an abnormal circumstance isdetected at a side of the transformer 1 having the discharge lamp 9, theprotection circuit 50 stops the operation of the logic circuit 29thereby preventing damages to the transformer 1 and relevant circuits.In this connection, the protection circuit 50 is adapted to stop theoperation of the logic circuit 29 only when a voltage exceeds a valuepredetermined by a built-in timer, whereby it is prevented fromhappening that the operation of the logic circuit 29 is falsely stoppedwhen an excess voltage is instantaneously applied for some reasons.

Referring to FIG. 7, the supply voltage Vcc is supplied to the step-upcircuit 100, the oscillating circuit 4, the PWM circuit 8, the erroramplifiers 11 and 51, the protection circuit 50, and the referencevoltage circuit 90. The supply voltage Vcc supplied to the referencevoltage circuit 90 is converted into lower reference voltages Vc and Ve,and the reference voltage Vc is inputted to the error amplifiers 11 and51, and the protection circuit 50 while the reference voltage Ve isinputted to the step-up circuit 100.

When the protection circuit 50 detects something abnormal at the side ofthe transformer 1 having the discharge lamp 9 connected, the logiccircuit 29 is caused to stop its operation thereby preventing damages tothe transformer 1 and relevant circuits. Especially, the H-bridgecircuit 17, to which the supply voltage Vb of 120V for lighting thedischarge lamp 9 is supplied, must be caused to infallibly stop itsoperation. In this regard, the protection circuit 50, when detectingsomething abnormal at the side of the transformer 1 provided with thedischarge lamp 9, stops the operation of the reference voltage circuit90 thereby reducing to a zero voltage the reference voltage Ve inputtedto the step-up circuit 100, which stops an output of the supply voltage76 supplied from the step-up circuit 100 to the logic circuit 29resulting in surely stopping the operation of the logic circuit 29.Consequently, the operation of the H-bridge circuit 17 can be reliablystopped without fail.

In the inverter circuit according to the present invention, the circuitsexcluding the H-bridge circuit 17, the transformer 1, and the dischargelamp 9 may be constituted by inverter control IC's.

While the present invention has been illustrated and explained withrespect to specific embodiments thereof, it is to be understood that thepresent invention is by no means limited thereto but encompasses allchanges and modifications that will become possible within the scope ofthe appended claims.

1. An inverter circuit for lighting discharge lamps, the invertercircuit comprising: a) a transformer having a resonant circuit formed bya parasitic capacitance of a discharge lamp; b) an H-bridge circuit todrive a primary side of the transformer at a frequency which is lessthan a series resonant frequency of the resonant circuit, and at whichphase difference in voltage and current at the primary side of thetransformer falls within a predetermined range from its minimum; c) alogic circuit to generate, based on an output signal of an oscillatingcircuit, gate signals for driving the H-bridge circuit; and d) a step-upcircuit to step up a DC supply voltage (Vcc) based on another outputsignal of the oscillating circuit, and to supply the logic circuit withthe stepped up DC supply voltage as a supply voltage for generating thegate signals.
 2. An inverter circuit for lighting discharge lampsaccording to claim 1, wherein the oscillating circuit is formed by theparasitic capacitance of the discharge lamp and an auxiliary capacitanceparallel-connected to the discharge lamp.
 3. An inverter circuit forlighting discharge lamps according to claim 1, wherein the step-upcircuit comprises an error amplifier to output a voltage in accordancewith an output voltage of the step-up circuit; and a PWM circuit tooutput a pulse voltage having a pulse width according to the voltageoutputted from the error amplifier based on the output signal from theoscillating circuit.
 4. An inverter circuit for lighting discharge lampsaccording to claim 3, wherein the step-up circuit further comprises aslow-start circuit connected to the PWM circuit.
 5. An inverter circuitfor lighting discharge lamps according to claim 4, wherein theslow-start circuit provided in the step-up circuit has a shorter risetime than a slow-start circuit to start the H-bridge circuit.
 6. Aninverter circuit for lighting discharge lamps according to claim 1,wherein the inverter circuit further comprises a protection circuit tostop an operation of the step-up circuit when detecting an abnormalcircumstance at a side of the transformer provided with the dischargelamp.